In hydrocarbon thermal processing plants, O-rings are used as mechanical seals for system pumps and compressors, as well as for tube fittings and pipe flanges to prevent leakage and failure.1 The national O-ring design guide by Federal-Mogul2 defines an 0-ring (i.e., a seal) as, “…a torus or donut-shaped part of circular cross section made of an elastomeric (rubber) material. O-rings function as low-cost, compact, reliable and forgiving sealing devices for liquids and gases. Because of the resilience of the elastomer, 0-rings absorb tolerance stack-up on the metal parts they seal (i.e., pack).”

An O-ring does this by blocking the gap between two surfaces by maintaining its shape under stress or returning to its shape when deformed. The thermal plant pump is normally conjured up when O-rings are discussed because a pump must generate enough volume (i.e., capacity) and pressure to circulate a heat transfer fluid (HTF) at the required rate. They must also be designed to specifications that enable them to operate at low and high temperatures. At temperatures above 200°C (390°F), a pump manufacturer will specify the fluid as well as the O-ring material to ensure they are compatible with the HTF.

Industry Industrial processes Type of HTF used in industry (some commercial examples)
Chemicals, plastics Plants-focused distillation; polymerization and cooling of reactions; esterification; gelatinizing channels; drying (e.g., drum driers, drying cabinets, drying channels); concentration of acids and lyes through evaporation; evaporators; calendars for foil production; automatic spraying, extruders and presses for plastic semiproducts; containers for dye and varnish industries; synthetic resin; for processing and transport of viscous substances that cannot be pumped at room temperature; heat recovery from flue gases following  combustion and reaction processes. Mineral-based HTF (e.g., BP Transcal N, Globaltherm M)

Synthetic based HTF (Therminol 66)

Food Large kitchens; pommes-frites; potato chips; fat hardening and rendering; bottle cleaning; can washing; spray drying for milk powder and blood powder production; roller driers; fat and oil vats; baking ovens; production of sweets and chocolate; starch drying; removal of smelly waste gases. Highly refined HTF (Marlotherm FG, Globaltherm FG)
Solar, energy source Concentrating solar power technologies including solar troughs; linear Fresnel reflectors and lens; dishes; towers. Synthetic based HTF (Dowtherm A, Globaltherm Omnitech)

Table 1. Examples of industrial processes using heat transfer fluids. Adapted from reference9

The Eastman Chemical Company highlights that above 200°C (390°F), a manufacturer will recommend that O-rings are: a) water-cooled or b) fluid-cooled or air-cooled, extended shaft seal and bearing.3 The Eastman Chemical Company and the national O-ring design guide by Federal-Mogul2 outlines further technical tips in the context of pump seals and leak prevention:

  1. Pumps with stuffing boxes should have five or more rings of laminar graphite packing.
  2. Use an insert gas (e.g., nitrogen) to blank an O-ring and avoid the depository effects of oxidation (the detrimental effects of oxidation have been discussed in a previous article).4
  3. Secondary seal should be used as additional safeguards against O-ring failures.
  4. Avoid unnecessary pipe support stresses placed on the main body of the pump because they can cause seal leakages.

Heat transfer materials

Industrial processing requires a constant heat source to produce consistent products. Many applications exist for heat transfer fluids, which reflects the fluids’ unique chemistries and a need for different fluids to be used in different processes (see Figure 1). The composition of high-temperature fluids can be organic (e.g., BP Transcal N, Shell Thermia B, Globatherm M), highly refined organic fluids (e.g., Marlotherm FG, Globatherm FG) or synthetic (e.g., Therminol 66, Dowtherm A, Therminol VP-1 and Globatherm Omnitech). The choice of fluid is affected by many factors including the cost, the sector, the safety of consumers5 and system requirements such as fluid traits (e.g., thermal stability, heat transfer efficiency6) and system design that encompasses the materials seals are made from.

Making buying decisions for O-rings

The composition of O-rings is heterogeneous, and at high-temperature operations these chemicals must be compatible with the heat transfer fluid. Hence it is important to select the proper elastomer for a specific O-ring application. Compromises may be necessary during this process, but it is normally possible to find a material that fulfils the requirements of the application. In almost all cases a manufacturer will define an elastomer material, then the first step will be to define the operating temperature of the O-ring elastomer (see Table 2) and its compatibility with a fluid.2 This will be followed by other factors concerning the elastomer (see list below) and the heat transfer fluid in question.

Maximum heat resistance Elastomer
327°C (621°F) Perfluoroelastomer (FFKM; common brand names include Kalrez, Chemraz, Isolast and Perlast)
300°C (572°F) Aflas (Tetrafluoroethelene propylene – TFE/P; a non-conventional fluoroelastomer (FKM) material that has a different chemical structure than other FKMs)
230°C (446°F) Fluorosilicone (FVMQ)
200°C (392°F) Fluorocarbon (FKM/FPM; commonly known Viton)
200°C (392°F) Silicone (includes compounds having vinyl-methyl-silicone (VMQ) as the main ingredient)
200°C (392°F) Viton Extreme
150°C (302°F) Hydrogenated Nitrile (Highly Saturated Nitrile Rubber [HNBR])
149°C (300°F) Ethylene Propylene (EPDM)
121°C (250°F) Hypalon (Chlorosulfonated polyethylene)
120°C (248°F) Neoprene (Chloroprene)
100°C (212°F) Nitrile (also known as Acrylonitrile-Butadiene (NBR) and Buna N)

Table 2. O-ring heat resistance guide. Adapted from reference10

Key mechanical properties of an elastomer7 to consider when making a buying decision may include:

  • Cost
  • Tensile strength
  • Maximum elongation
  • Minimum and maximum hardness
  • Resilience/rebound
  • Compression set
  • Adhesion to metals
  • Resistance to abrasion, tear, weather, ozone, water swell, steam, flame, chemical, acid and alkali
  • Properties concerning dynamics and electrical
  • Gas permeability

Elastomers & well-known heat transfer fluids

A wealth of online data exists concerning the compatibility of O-rings and HTFs, and the reader should consult this resource as an example for specific fluids7. The compatibility of Therminol 66, Therminol VP-1 and Dowtherm A, all synthetic fluids, with the various elastomers listed in Table 2 has been reported. Table 3 summarizes the strength of available evidence. All three fluids show the highest compatibility rating (a score of 1) with Kalrez O-rings. Viton O-rings also score highest for Therminol VP-1 and Dowtherm A, but not Therminol 66.

For Therminol VP-1 and Dowtherm A, inconsistent data have been reported for silicone (scores of 2 [fair evidence] and 4 [unsatisfactory], respectively) and fluorosilicone O-rings (scores of 5 [insufficient] and 2 [fair], respectively). As the composition and upper operating temperatures are equivalent for these two fluids, it would seem that differences may exist because of plant design or business collaborations between the specific O-ring providers and fluid manufacturers. The latter point would seem to be supported by the lack of evidence for Therminol 66 and all elastomers except Kalrez O-Rings (see Table 3).

Chris Wright, heat transfer fluid, seals

Table 3. Evidence for the compatibility of elastomers and Therminol 66, Therminol VP-1 and Dowtherm A. Table adapted from mykin.com/rubber-chemical-resistance-chart. Supporting evidence is rated satisfactory (1) to insufficient (5).

Mechanical seals — What else to consider

When selecting a seal, define the maximum temperature for the seal and the HTF. Indeed, in cases where the HTF maximum temperature is exceeded but the maximum seal temperature is not, the potential exists for the seal to coke, which can cause the O-ring to fail.8

As highlighted above, heat (and heat expansion) can affect the plated face of the O-ring and lead to cracking and subsequent failure of the O-ring because of elastomer degradation. It is also well-known that heat increases the thermal degradation of all HTFs through increased reaction rates, which can also lead to O-ring failures. Moreover, many products will change from a liquid to a solid or gas at high temperature, which can lead to seal faces being “blown open” and O-ring failures.

It is important that all seals are compatible with any fluid pumped through a system, and this includes the high-temperature HTF and the flushing/cleaning fluids used during the initial cleaning of a plant or when flushing that follows the replacement of degraded high-temperature HTF.

It is a given that all HTFs will thermally degrade when operating continuously at high temperature. This leads to the formation of “light” and “heavy” chain hydrocarbons, which change the composition of an HTF. Moreover, the thermal degradation of a fluid in the presence of oxygen will lead to acidification of the HTF. The interaction of these chemicals with O-rings needs to be considered when making a buying decision and also as part of an ongoing HTF maintenance plan that can be used to monitor the fluid throughout its lifecycle.

When researching an O-ring, it is advisable to work with reputable suppliers and manufacturers. They should be able to provide O-ring charts that detail the elastomer properties and characteristics. Then before making a decision, it is recommended that you contact the manufacturer for technical advice and to discuss the application and to confirm the seal specifications.

Main take-home messages

  • Define the operating temperature of the seal and its compatibility with a fluid.
  • For well-known synthetic high-temperature HTFs (Therminol 66, Therminol VP-1 and Dowtherm A), Kalrez O-rings have the strongest evidence to support their use at continuous high temperature.
  • Consider the compatibility of the O-ring and fluids used to clean and flush a system.
  • All high HTFs will thermally degrade over time and/or oxidize in the presence of oxygen and these chemicals can affect an O-ring’s stability. Routine management of HTFs is recommended to monitor and correct any changes in the chemistry of the fluid. Likewise, a high-purity HTF is advised to reduce the potential effect of any impurities in the fluid.
  • Before buying an O-ring, consult a reputable supplier/manufacturer regarding its application and to confirm its technical specifications.

References

  1. Bloch H.P. & Geitner F.K. Practical machinery management for process plants. Machinery component maintenance and repair. Volume 3; 586-601. Gulf Professional Publishing, part of Elsevier.
  2. Federal-Mogul. National O-Ring design guide. Retrieved from http://www.allsealsinc.com/pdf/National%20O-Ring%20Catalogue-1.pdf. Accessed: Feb. 25, 2017.
  3. HTF system equipment. Online resources by Therminol heat transfer fluids by Eastman. Retrieved from https://www.therminol.com/resources/heat-transfer-system-equipment. Accessed: Feb. 25, 2017.
  4. Wright, C. Monitoring the condition of a heat transfer fluid has pump benefits. Retrieved from http://www.flowcontrolnetwork.com/monitoring-the-condition-of-a-heat-transfer-fluid-has-pump-benefits/. Accessed: Feb. 25, 2017.
  5. Wright, C., Bembridge T., Picot, E. & Premel, J. Food processing: the use of non-fouling food grade heat transfer fluids. Applied Thermal Engineering 2015: 84; 94-103. Source: http://www.sciencedirect.com/science/article/pii/S1359431115002562.
  6. Wright, C. What to consider when making the buying decision about a heat transfer fluid for your system: A report of the webinar hosted by Process Heating. Journal of Applied Mechanical Engineering 2016: 5 (1); http://dx.doi.org/10.4172/2168-9873.1000191.
  7. Mykin Inc. Rubber Properties. Retrieved from http://mykin.com/rubber-properties Accessed: Feb. 25, 2017.
  8. McNally Institute. A few rules of thumb for mechanical seals. Retrieved from http://www.mcnallyinstitute.com/02-html/2-5.html. Accessed: Feb. 25, 2017.
  9. Wright C. Managing mineral-based heat transfer fluids to help maintain a safe and effective heat transfer plant. International Journal of Materials Chemistry and Physics 2015: 1 (3); 246-252. Retrieved from http://files.aiscience.org/journal/article/html/70130040.html
  10. Fluid seals – O-Ring application guide. Retrieved from http://www.fluidseals.com.au/o-ring_application_guide.php. Accessed: Feb. 25, 2017.

Author’s note: The author would like to acknowledge the writing support provided by Red Pharm communications, which is part of the Red Pharm company. Visit @RedPharmCo on Twitter.

Chris Wright is a research scientist for Global Group of Companies. He graduated from the University of Leeds in the U.K. with Bachelor of Science and doctorate degrees. His research focuses on the use and maintenance of heat transfer fluids in manufacturing and processing, which includes food, pharmaceutical, specialist chemical and solar sectors. Please contact the author for reference materials cited in this article. He may be reached at chrisw@globalgroup.org.